This invention relates to viscosimeters which provide a representation of the viscosity of those fluid portions adjacent to the viscosimeter and particularly to viscosimeters wherein a transducer is used to determine this viscosity by its imparting shear waves to the adjacent fluid portions.
Various viscosimeters using members in motion to impart shear waves to fluids to determine the viscosity thereof have been devised heretofore. Among them is a viscosimeter in which a cup, suspended by wires, is driven in a rotational oscillation of the cup to provide an indication of the viscosity of the fluid. This viscosimeter requires rather an elaborate mechanical arrangement and typically uses an elaborate electrical arrangement utilizing a feedback loop from one suspension wire to the driver which loop includes amplification and other operations on the signals involved.
Another viscosimeter utilizes a pair of rectangular bars, one of the bars at least being of a piezoelectric material, with the fluid of interest located therebetween. The piezoelectric bar receives shear waves transmitted through the fluid by the other bar when this other bar is driven in reciprocating, oscillatory motion in its direction of elongation. The motion of the piezoelectric bar as a result of the shear waves impinging thereon is an indication of the viscosity of the fluid, this motion being converted into an electrical output by the transduction of energy from one form to another occurring in the piezoelectric bar. Again, a rather elaborate mechanical arrangement is required. A rather small output signal is obtained which usually must be amplified in the electrical output circuit for the signal to be satisfactorily used.
A viscosimeter which, in mechanical principles, is somewhat simpler than the foregoing viscosimeters is based on an electrically driven, torsionally vibration, cylindrical piezoelectric crystal. These torsional vibrations impart primarily shear waves to those fluid portions adjacent to the crystal with use of a properly designed crystal. The effects of the fluid at the surface of the vibrating crystal provides a damping force on the crystal, i.e., the loading thereon. Therefore, as the viscosity changes and so the damping at the crystal's surface, the effective input impedance seen at the electrical driving terminals of the crystal also changes. The effective electrical input impedance can be analytically derived from an equivalent electrical circuit which includes in its generalized impedances the mechanical effects of both the torsionally vibrating crystal and the fluid. Viscosity can be determined through measuring the input impedance at the crystal resonance frequency with another calibration measurement of the input impedance with the crystal submerged in a fluid of known parameters. This impedance measurement, however, is a rather inconvenient measurement.